This competing renewal application builds on the success and productivity of our current project investigating changes in the pneumococcal population in the conjugate vaccine era, and seeks to continue surveillance and analysis during the introduction of PCV13. Our work to date has substantially increased our understanding of how this pathogen responds to the potent selective pressure of a vaccine. As PCV7 vaccine serotypes have virtually disappeared from the nasopharynx, they have been quickly and completely replaced by non-vaccine serotypes. Clinically, these non-vaccine serotypes have become responsible for the great majority of invasive pneumococcal disease. Now, a new 13-valent pneumococcal conjugate vaccine (PCV13) that includes 6 additional serotypes (1, 3, 5, 6A, 7F, 19A), will be introduced in 2010. This research will provide data needed to understand the clinical implications of PCV13 use, test specific hypotheses we have developed about bacterial adaptation, and serve as a template for understanding the evolution of other pathogens. Given our team of collaborators, the ongoing commitment of community partners, and our access to state-of-the-art genetic sequencing resources, we are uniquely positioned to address the following specific aims: 1. To examine trends in pneumococcal colonizing and invasive disease isolates, with regard to serotype and antibiotic resistance, host risk factors, and invasive potential, before and after introduction of PCV13. 2. To assess shifts in pneumococcal population structure (by MLST) and evaluate potential factors associated with successful spread of pneumococcal clones in Massachusetts in the context of PCV13 introduction. 3. To use whole genome sequencing to identify potential genetic determinants associated with serotype switching and invasiveness among clones that have emerged under selective vaccine pressure. To achieve these goals, we will collect new nasopharyngeal specimens from 2,250 children as they present for routine pediatric care (in 2011 and 2014) in nine distinct Massachusetts communities, and analyze them in the context of previous collections available for comparison from 2001, 2004, 2007, and 2009. In addition we will simultaneously analyze invasive disease isolates collected from children in Massachusetts as part of an enhanced statewide surveillance program in Massachusetts since 2001. In total, this continuing project provides an unprecedented opportunity to assess bacterial evolution in real time, and connect changes in carriage to those in invasive disease.